In recent years, studies on medical materials utilizing various polymer materials have been progressed, and application to a blood filter, an artificial kidney membrane, a plasma separation membrane, a catheter, an artificial lung membrane, an artificial blood vessel, an adhesion-proof membrane, an artificial skin, and the like have been expected. Since this case results in using a synthetic material that is a foreign matter for a living body by bringing it in contact with tissues in a living body and blood, it is required for the medical material to have biocompatibility.
When the medical material is used as a material being in contact with blood, three elements including (a) suppression of the blood coagulation system, (b) suppression of adhesion and activation of platelets and (c) suppression of activation of the complement system become important points for biocompatibility. In particular, in the case of using the medical material as a material being in contact with blood for a comparatively short time such as an extracorporeal circulation medical material (for example, artificial kidney and plasma separation membrane), generally, for simultaneously using an anticoagulant such as heparin or sodium citrate, suppression of activation of platelets and the complement system of the above described (b) and (c) is a particularly important object.
Regarding (b) suppression of adhesion and activation of platelets, it is considered that a micro phase-separated surface, or a hydrophilic surface, particularly, a gelled surface with which a water-soluble polymer is combined is excellent, and a hydrophobic surface such as that of polypropylene is said to be inferior (Nonpatent Documents 1 and 2). However, although a surface having a micro phase separation structure makes it possible to exhibit good blood compatibility by controlling the surface itself to an appropriate phase separated state, conditions capable of producing such phase separation are limited and there is a restriction on applications thereof. Further, in a gelled surface with which a water-soluble polymer is combined, although adhesion of platelets is suppressed, platelets activated on a material surface and small thrombus are returned to a body, and variation of abnormal blood cell components (platelets) are frequently observed, which may cause a problem.
On the other hand, (c) regarding activation of the complement system, it is known that a surface having a hydroxyl group such as cellulose or an ethylene-vinyl alcohol copolymer has high activity, and a hydrophobic surface such as that of polypropylene has slight activity (Nonpatent Document 3). Therefore, when a cellulose-based or vinyl alcohol-based material is used in, for example, an artificial organ membrane, a problem of activation of the complement system is caused, and on the contrary, when a hydrophobic surface such as that of polyethylene is used, a problem of adhesion and activation of platelets is caused.
Further, in the case of using the medical material as a material being in contact with blood for a comparatively long time, for example, such as an artificial blood vessel, in addition to the above described three elements, in order to preferably carry out neointimal formation and regeneration and reproduction of tissues in a living body, it is necessary to use a material having an affinity for tissues (cells) in a living body. As such a material of an artificial blood vessel, for example, an artificial blood vessel made of a super-micro polyester fiber is known (Nonpatent Document 4). This super-micro polyester fiber is one of medical materials utilizing recognition of foreign matters in a living body, wound healing by biological defense, and self tissue reproduction, and is mainly used as an artificial blood vessel at present. However, long term application of this artificial blood vessel to a fine blood vessel causes a problem of occluding the artificial blood vessel.
Further, for medical devices being in contact with tissues in a living body or a biological fluid other than blood, for example, adhesion preventing membranes and implant materials that are used by being embedded in a living body for a long time, or wound dressing materials used in contact with wounds (regions of injury caused by peeling of a skin and exposition of a living tissue), it is required to have a surface with less recognition of foreign matters in a living body and easily separating from the living body (non-adhesive surface). However, since tissues in a living body are adhered to a material surface in the cases of silicone, polyurethane and polytetrafluoroethylene conventionally used as the above described materials, recognition of foreign matters in a living body is too strong, and thus, satisfactory performance could not be obtained.
Another medical material is polyethylene glycol (PEG). PEG has significantly excellent blood compatibility, and many application studies for the medical field have been made. However, since PEG is water-soluble, in the case of being used as a medical material, PEG has been required to be formed into a block copolymer or a graft copolymer with other polymers and fixed to a material surface.
Further, a technique of exhibiting antithrombogenicity by coating a surface being in contact with blood with poly(2-methoxyethyl acrylate) that is a biocompatible material has been known (Patent Document 1). However, since the material dissolves in methanol but insoluble in ethanol, toxicity of a solvent remaining in a substrate after coating is concerned.
Further, a water-soluble copolymer of polyethylene glycol acrylate and alkyl acrylate has been known (Patent Document 2). With this technique, protection of a surface of a solid phase can be performed in immunoassay. However, since this copolymer is water-soluble, long-term maintenance of biocompatibility is difficult.
Furthermore, a phosphorylcholine analog group-containing polymer has been known (Patent Document 3). This technique allows a phosphorylcholine analog group-containing polymer to be water-insoluble while maintaining good biocompatibility by copolymerizing a hydrophilic (meth)acrylate monomer (hereinafter referred to as MPC) containing a phosphorylcholine group, which has high biocompatibility, and an alkyl(meth)acrylate monomer having high hydrophobicity. However, since this copolymer is in a rigid solid state, there is not only a possibility of separation of a film after coating, but also insufficiency of biocompatibility from the viewpoint of immunity. It is considered that an ammonium ion (N+) contained in a phosphorylcholine group activates the immune system in a living body (see FIG. 1).
(Nonpatent Documents)
                1. Trans. Am. Soc. Artif. Intern. Organs, vol. XXXIII, p. 75-84 (1987)        2. Polymer and Medical Treatment, Mita Publishing Company, p. 72-73 (1989)        3. Jinkou Zouki, 16(2), p. 1045-1050 (1987)        4. Jinkou Zouki, 19(3), p. 1287-1291 (1990)(Patent Documents)        1. Japanese Patent Application Laid-Open (JP-A) No. 2002-105136        2. Japanese Patent Application Laid-Open (JP-A) No. 287802/99        3. Japanese Patent Application Laid-Open (JP-A) No. 35605/99        
As a method for producing the copolymer of polyethylene glycol acrylate and alkyl acrylate, reprecipitation is most conveniently used. Herein, reprecipitation refers to a technique of repeatedly generating precipitates to enhance the purity, and is performed for the purpose of removing low molecular weight compounds (such as monomers) in polymer synthesis. However, since two types of monomers being hydrophilic and hydrophobic are used in the present invention, selection of a poor solvent used in reprecipitation is important. That is, this is because that an ability of dissolving both of the monomers having different properties and precipitating only the obtained copolymer is to be required. However, it has been difficult that a sieve of such a subtle dissolving property is exhibited with one solvent.